Large scale storage systems have traditionally been based on arrays of hard disks. These disks have a relatively high failure rate and, therefore, it is important that they can be quickly and easily replaced, especially for an on-line data storage system where the stored information is often required to be immediately accessible. For example, Enterprise-class data requires on-line access, high performance, high bandwidth, high availability and high throughput. ‘Hot plug’ technology has been developed to provide the above mentioned requirements together with easy disk replacement whilst the storage system is still active. This type of data storage technology, however, is very expensive and, therefore, is only appropriate for high value data that must be immediately accessible.
RAID (originally “Redundant Arrays of Inexpensive Disks”, now “Redundant Arrays of Independent Disks) offers the ability to store data in arrays of disks, a data set being distributed over an array of disks by a RAID controller. RAID may be configured with parity such that it in the event of a failure of one of the disks in the array controlled by a RAID controller, the data on the failed disk can still be recovered from the remaining disks. RAID provides a method of accessing multiple individual disks as if the array were one large disk, spreading data access out over these multiple disks, thereby improving access times. By placing data on multiple disks, input/output (I/O) operations can overlap in a balanced way, improving performance.
Typically RAID is used in large file servers, transaction or application servers, where data accessibility is critical, and fault tolerance is required. RAID is also being used in desktop systems for computer aided design (CAD), multimedia editing and playback where higher transfer rates are needed.
Incorporation by reference is made to “A Case for Redundant Arrays of Inexpensive Disks (RAID)” by Patterson, Gibson and Katz as presented at ACM SIGMOD Conference, June 1998.
Whilst not within the scope of RAID, data can also be geographically distributed to remote arrays and storage systems spanning disk arrays and automated libraries of optical disks and tapes controlled by a RAID or RAID-like controller. Thus, the storage media to which data is distributed (and from which it is retrievable) need not be homogeneous.
An array of disks can also be provided without a RAID controller, being known as JBOD Oust a bunch of disks).
The disks have a fairly high failure rate so a high priority in the design of storage systems is to make replacement of failed disks quick and easy. This has led to the development of hot plug technology which allows a failed disk under a RAID controller to be physically replaced without requiring the other disks in the array to be taken off-line or powered down. Because of RAID's built-in fault tolerance/redundancy, upon failure of a disk, the data on the failed disk is recoverable from the remaining disks. The controller highlights the failed disk and calls for replacement which is effected manually by unplugging the failed disk from its mount and plugging in a fresh disk—also a hot plug disk.
Hot plugging works very well for on-line data and provides maximum data protection, performance and availability and is ideally suited to Enterprise-class data where the ready availability of the data is regarded as having high value.
As previously noted, any hot plug drive, in addition to the basic hard disk, must include a mechanical chassis including a back plane that provides for simple insertion and removal from a specially adapted and tailored slot in a hot plug storage array enclosure. Access to the enclosure, typically, in a standard data centre equipment 19″ rack (such as the short standard rack shown in FIG. 1 of the accompanying drawings) is from the front of the rack to allow access to the disks and enable them to be taken out while the system is on-line. The disks each have a finger ring pull to allow them to be individually drawn out and extracted from the front of the rack. In practice, there is no standard design for these components such that each manufacturer devises their own design, making it essential that the manufacturer of the enclosure supplies the hot plug disks. This lack of standardisation reduces competition and economies of scale thereby inflating the price of the disks.
Referring to FIG. 2 of the accompanying drawings, this shows four hot plug disks in a JBOD array in a rack. Three hot plug disks have been removed from the array to expose the back plane electronics. Behind the back plane electronics are one or more power supply units. Referring to FIG. 3 of the accompanying drawings, the boundary of the hot plug disks and their associated electronics is shown by the dashed lines from which it will be appreciated that the disk array does not extend to the rear of the rack so that there is significant unused space toward the rear of the rack. This arrangement showing the unused space behind a hot plug disk array is also shown schematically in FIG. 4 of the accompanying drawings.
Many design constraints arise from the need to provide hot plug capability and all serve to increase costs. For example, the metal work design, locking mechanisms, interface design and back plane electronics are all determined by the necessity to have hot plug technology.